[RFC] remoting KMS

[Daniel Vetter]

On Thu, May 17, 2018 at 8:23 PM, Thomas Hellstrom <thomas at shipmail.org> wrote:
> Hi!
>
> I'm currently working on a remoting KMS backend, and now I thought it would
> be a good time to get some feedback on a very rough design:
>
> The idea is that we want to be get enough information on the backend side of
> KMS to be able to remote the display system over VNC or something similar.
> Initially I'd plug in an open-source VNC server as the "reference"
> user-space.
>
> The solution should be general enough to plug into any driver, including
> VKMS, either mirroring an existing crtc/connector or adding a virtual one.
> The driver would typically only have to implement an efficient readback
> command, optionally with a diff-map or damage rect that tracks changed
> content for the VNC server to be able to do its work efficiently.
>
> This would all be forwarded to a user-space interface and protocol with a
> slightly more elaborate user-space event-mechanism than the one we have in
> drm now. In particular we want to be able to read partial events, and resume
> event reading after a partial event. We would also want to be able to send
> commands either through an ioctl() or using write(). Current test code is
> using write().
>
> Since the VNC server in our application typically will be system-wide, we
> need to resurrect a control-node functionality. I was trying to use sysfs
> attributes for this, but while sysfs supports rudimentary poll
> functionality. It's a bit too rudimentary and tracking client lifetime
> becomes racy. Perhaps control nodes renamed to "remoting" nodes or something
> similar. It might be that masters want to attach a VNC server to their
> current display and in that case we probably would want to expand primary
> nodes with this functionality as well.
>
> Now to the open-source  problem. As mentioned, the reference user-space will
> be a solution based on libvncserver. In fact there will be a small
> user-space interface library that libvncserver can interact with, similar to
> libdrm. However, I envision in the long run VMWare would be interested to
> use other remoting protocols with closed source protocol encoders, using the
> open source reference library. In theory, this solution would also allow for
> simple (open- or closed source) user-space display drivers, very similar to
> DisplayLink's evdi solution, even if that's not a use-case VMWare is
> currently interested in.
>
> Thoughts and comments appreciated.

Just two very quick comments:

- For the uapi - can't we just use v4l for the output side? That gives
you buffer handoff, syncing (v4l is gaining dma_fence support) and
pretty much everything you might want. And we've talked about building
kms->v4l pipelines in a bunch of other use-cases already (like feeding
into video encoders for widi and stuff like that). Would give you the
userspace for free, only thing we really need is to (maybe, not sure
you'd even need that) expose what kms output corresponds to what v4l
pipeline.

- You say "The solution should be general enough to plug into any
driver" - is the idea that any hw driver would automatically support
this? Given that prime-like setups are getting ever more common
(mobile is essentially all prime with display and offloaded rendering)
I think it'd be cleanest if we don't tie that virtual output to other
drivers, but bind things together using buffer sharing/dma-fences and
prime. Adding v4l output to vkms is actually something we've already
discussed a bit on irc, but with the use-case that you could just
watch what your desktop/window manager is doing under a specific
simulated setup (e.g. pll sharing constraints, or special plane
features, or whatever other specific thing you want to test your
compositor against but isn't supported on all physical hw/drivers).

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch